1. Field of the Invention
The present invention relates to horizontal beam deflection in television and video display monitors, particularly to raster shifting whereby the horizontal position of a video raster on a display screen is adjusted.
2. State of the Art
In television and video display monitors using a CRT (cathode ray tube), a horizontal deflection current is produced in a horizontal deflection coil to create a magnetic field used to deflect an electron beam (produced by a cathode of the CRT) back and forth across a display screen. As the electron beam is scanned back and forth across the display screen, it is modulated to produce on the display screen spots of varying luminous intensity, thus forming an image to be viewed. A high voltage, referred to as EHT (electrical high tension), is applied to an anode of the CRT and accelerates the electron beam, causing it to strike the screen at a very high speed.
Typically, a periodic horizontal drive signal is applied to a horizontal scan output stage (i.e., a power transistor) to produce a train of high-voltage retrace pulses used to generate the horizontal retrace current within a horizontal deflection, or DY, coil. The DY coil is part of a deflection yoke of the CRT, a combination of coil windings formed so as to surround particularly a neck portion of the CRT.
Because of manufacturing uncertainties, the yoke and CRT gun system may exhibit symmetry problems. To correct these problems, raster shifting may be used to adjust the horizontal position of the video raster on the display screen. In practice, all CRTs require some amount of raster shift. For example, symmetry problems are often caused by a slanted gun mounting inside the tube, but also occur in the convergence and beam landing adjustment process.
In existing designs that provide for raster shifting, basically a DC current is injected through a inductor into the deflection yoke. The DC current is generated from a floating power supply output. The polarity and amount of raster shift is usually only manually adjustable.
Deflection yokes used for high resolution monitors tend to have very low impedance, which reduces the trace voltage but increases the scan current. In order to provide the necessary raster shift, a DC current of 1A must be provided, which requires a drive voltage of only about 0.8 V.
Modern power supplies designed for high efficiency, however, generate 3 to 4 volts per turn, which causes 3 to 4 V to be the lowest possible supply voltage. In known raster shifting circuits, when the required 1A current is supplied, high power losses are inevitable because the lowest output voltage which can be generated by the power supply is unavoidably a few times higher then needed. If, on the other hand, a non switching approach is used, a complicated drive circuit is required to make the current controllable and the H-scan rate dependable.